ABSTRACT Severe Maternal Morbidity (SMM) includes serious threats to maternal health and survival that occur at delivery or postpartum. Based on the SMM index developed by CDC that focuses on the most life-threatening conditions and complications, the prevalence of SMM doubled from 1998-2011 and currently affects >65,000 women in the U.S. each year. Maternal health is essential to a woman?s ability to care for her children and to her health over her life course, yet our understanding of causes of SMM is limited. This proposal addresses three key and understudied areas of research for SMM: racial/ethnic disparities, maternal weight, and social disadvantage. Racial/ethnic disparities in SMM and its contributing conditions are well known ? risk of SMM tends to be up to 2-fold higher among non-Hispanic blacks and 1.5-fold higher among Hispanics and Asians, relative to non-Hispanic whites. The explanation for these disparities is unknown, but preliminary evidence suggests that maternal weight and social disadvantage may contribute. In the U.S., 25% of women are obese at conception, half gain excessive weight during pregnancy, and 25% retain >10 lb. postpartum, but this varies by race-ethnicity. Some studies suggest an association of obesity with SMM, but few have examined severity of obesity, underweight status, or weight change. Social disadvantage is much more common among non-whites than whites and associated with myriad perinatal outcomes, but almost no studies have examined its contribution to SMM. Unusually high and low maternal weight and weight gain are most likely among socially disadvantaged women, but their inter-related impacts on SMM have not been studied. Our goal is to increase understanding of biologic and social determinants of SMM and its racial/ethnic disparities, by analyzing 4 million births that occurred in California from 2007-2014. The data include vital records and mother and infant hospital discharge data from pregnancy through postpartum. Outcomes will include the most common conditions that contribute to SMM ? postpartum hemorrhage, eclampsia/severe preeclampsia, select cardiovascular conditions, and sepsis ? as well as the SMM index developed by CDC. The Specific Aims are: 1) Examine associations of maternal weight status before, during, and between pregnancies with SMM and whether they are modified by race/ethnicity;? 2) Examine associations of multi-level (individual, household, neighborhood) indicators of social disadvantage with SMM, overall and by race/ethnicity (Aim 2a), and the extent to which these relationships are mediated by maternal weight status (Aim 2b);? and 3) Investigate the contribution of maternal weight status and social disadvantage to the population burden of SMM and its racial/ethnic disparities. The proposed research will break new ground by studying the complex relationships among maternal weight, social disadvantage, racial/ethnic disparities, and SMM. This knowledge is essential to the development of effective public health and clinical interventions to reduce SMM and its disparities, including guidelines for maternal weight.